Multiple field units, which are used to record and/or control process variables, are utilized in process automation engineering. Examples of field units of this type are level measurement devices, mass flow meters, pressure and temperature measurement devices, pH and redox potential meters, conductivity meters etc., which, as sensors, record the corresponding process variables such as fill level, flow rate, pressure, temperature, pH value and conductivity value.
To influence process variables, so-called actuators are utilized, e. g. valves that control the flow rate of a liquid in a pipeline section, or pumps that change the fill level in a container.
A variety of such field units are manufactured and marketed by the company Endress+Hauser®.
Such field units are often connected to superordinate units, e. g. control systems or control units. These superordinate units are utilized for process control, process visualization or process monitoring.
The energy and/or signal transmission between field units and superordinate units frequently takes place according to the known 4 to 20 mA standard, where a 4 to 20 mA current loop or two-wire line runs between the field unit and the superordinate unit.
The HART protocol is based on this 4 to 20 mA standard. With the aid of the HART protocol and the thus enabled bidirectional communication, the aforementioned field units can be very flexibly parameterized and put into operation, and data records stored on the device can be easily retrieved.
Consequently, the HART protocol enables a very simple operation of the field unit through a control unit.
To this end, the data to be transmitted are modulated onto the 4 to 20 mA signal or the 4 to 20 mA current loop to ensure digital communication between the field unit and the control unit, with the analog signal transmission taking place simultaneously.
If, for example, the field units are sensors, the measured values recorded by the sensors are transmitted in the form of an analog current signal to the superordinate unit via the two-wire line. The measuring range of the sensors is represented linearly on the 4 to 20 mA current signal. At the same time, a two-way communication is available through the digital HART communication, for example to carry out a parameterization of the field unit using the control unit. In addition to the HART protocol, there are other protocols such as Foundation Fieldbus and/or Profibus PA that provide functions similar to those of HART.
The energy supply to the field units likewise occurs through the 4 to 20 mA current signal so that no other supply line is needed in addition to the two-wire line. Normally, the energy supply is not realized through the superordinate unit, but through a measuring transducer supply unit that is connected to the 4 to 20 mA current loop or two-wire line and typically arranged separately from the superordinate unit.
For the use of field units in explosion-hazardous areas, specific measures regarding intrinsic safety are necessary. Thus, a galvanic isolation is needed for the transmission of signals between the superordinate unit, which is located in the safety zone, and the field unit, which is located in the intrinsically safe zone.
From state of the art technology, control units are known that are physically connected to the 4 to 20 mA current loop for communication with the field unit, thus enabling the field unit to be controlled by means of the HART protocol through the 4 to 20 mA current loop. Such control units have the disadvantage, as stated above, that they need to be physically connected or tapped to the 4 to 20 mA current loop. To that end, the two-wire line has to be modified, e. g. stripped and/or cut, to enable a connection to the control unit. As a result, the actual analog current signal that transmits the measured values of the field unit is not transmitted during the connection process of the control unit. In other words, the analog transmission of the measured values is interrupted.
It is also known from state-of-the-art technology that radio modules are utilized to facilitate the operation of field units. Solutions are known, where field units have integrated radio modules. However, it has also become known that field units have been retrofitted with radio modules, so that the field units then can be operated by utilizing control units that are likewise equipped with a radio unit.
Such solutions have the disadvantage that the radio module requires an energy supply in addition to the field unit. The energy supply to the field units and to the radio module is achieved, as mentioned above, through the 4 to 20 mA current loop. The supply of electrical energy to the 4 to 20 mA current loop takes place e.g. through a measuring transducer supply unit, as described above. This provides the electrical voltage required for the adjustment of a current value in the current loop. Often, the energy that can be supplied to the current loop when using the signal line is not sufficient to provide the field unit with the energy needed for operating the integrated radio module.
A remedy for this disadvantage is found, as proposed in German patent, DE 10 2004 020 393 A, in providing a separate energy supply for the radio module in the form of a battery.
However, the disadvantage of this solution lies in the fact that the maintenance, especially monitoring the battery status, requires a substantial effort from the operator of such a field unit.
Moreover, an additional disadvantage of such solutions is found in the fact that field units fitted with radio modules are not permanently controllable, which means that inadvertent operation or even deliberate manipulation cannot be ruled out or prevented. Thus, for example, it can occur that an operator, who is within reach of the radio module for wireless communication, can inadvertently be connected to the field unit and thus trigger an unintended operation of this field unit. In addition to such unintended operations, deliberate actions can be carried out in the form of manipulation of such field units that are fitted with radio modules. Especially in large automation plants, where the field units are spread out over a wide area, effective control of the field units can be extremely difficult, and it cannot be ruled out that manipulations of field units occur, e. g. in the form of a hacking attack.